N–H cleavage as a route to new pincer complexes of high-valent rhenium

Abstract

A novel PNN-type pincer ligand has been accessed via imine reduction with LiAlH₄ to provide the phosphine diamino proligand, PN^HN^H (1). The ligand, 1, as well as PN^HP can be metallated directly, via N–H cleavage, with L₂ReOX₂(OEt) precursors to access six-coordinate (PNP)ReOCl₂ (2) and (PNN^H)ReOX₂ (3-Cl, X = Cl; 3-Br, X = Br) in good yield. 3-Cl and 3-Br undergo dehydrohalogenation upon treatment with NEt₃, furnishing the five-coordinate phosphine/diamido PNN-type compounds (PNN)ReOX (4-Cl, X = Cl; 4-Br, X = Br) in excellent yield, presenting as a mixture of rotameric diasteromers. The reversibility of this deprotonation, and the coordinative unsaturation of 4-Cl is shown in reactions with HCl_(aq) and PMe₃ providing 3-Cl and 4-PMe₃, respectively. Treatment of 4-Cl with AgOAc, AgOTf, or NaHBEt₃ lead to formation of (PNN)ReO(OAc) (4-OAc), (PNN)ReO(OTf) (4-OTf), and (PNN)ReO(H) (4-H), all isolated in excellent yields in varying diasteromeric ratios. The nature of the isomerism was analyzed based on solid-state structural studies and solution NMR data.